Abstract Collapsing glomerulopathy is a severe form of human glomerular disease that frequently results in rapid progression towards end stage kidney failure. Whereas this disease can be caused by HIV infection and other causes, this proposal will focus on a specific form of this disease caused by circulating proteins (hereon referred to as CG). Although currently classified as a form of focal and segmental glomerulosclerosis(FSGS), there are several molecular and morphological differences that set CG apart from other forms of FSGS. The applicants developed the first ever animal model of CG by injecting plasma from CG patients into a specific stain of inbred Sprague Dawley rats (IMC rats) over a decade ago. Further exploration of this model, and correlation of its molecular changes with human kidney biopsies resulted in the development of new mechanism based models that will be utilized in this proposal. The proposal is structured to explore two distinct morphological aspects of CG, epithelial cell proliferation and glomerular capillary loop collapse, at a molecular level and also explore cross talk between these two components of CG. Finally, a proteomic approach will be used to identify and create a short list of circulating proteins that cause this disease. De-identified human plasma and controls, and plasmapheresis samples from patients with recurrent CG after transplantation (referred to as CG plasma) will be used for these studies. The overall goal of the proposed studies is to develop new insight into the molecular pathogenesis of CG so as to be able to develop new therapies for this disease. In Specific Aim 1, molecular mechanisms of podocyte proliferation induced by ZHX2 and TERT will be explored using podocyte specific ZHX2 transgenic rats and TERT knockout rats in the IMC background injected with CG plasma. In Specific Aim 2, a new model of CG developed in ?5 integrin knockout mice will be used to study the role of ?3 integrin in the development of glomerular capillary loop collapse. Mice deficient in ?3 integrin and podocyte specific ?3 integrin knockout mice will be used to study the role of ?5 integrin in CG. Rrm2b and ?5 integrin knockout mice will be interbred to study pathway synergies. Finally, crosstalk studies between the podocyte and the endothelium during the pathogenesis of CG will be explored. In Specific Aim 3, CG and control plasma will be depleted of 14 common plasma proteins, their ability to induce CG changes confirmed, and the putative circulating proteins identified using a proteomics approach comprising of DIGE and nano-LC MS/MS. A short list of proteins will be tested in vitro and in vivo using the above animal models for their ability to induce CG.